The skin

The skin is by far the largest organ of the human body, weighing about 10 pounds (4.5 kg) and measuring about 20 square feet (2 square meters) in surface area. It forms the outer covering for the entire body and protects the internal tissues from the external environment.

The skin consists of two distinct layers: the epidermis and the dermis. Each layer is made of distinct tissues and performs distinct functions to support the body. A third layer of tissue under the skin, known as the hypodermis or subcutaneous layer, is not truly part of the skin itself but connects the skin loosely to the underlying muscles and bones that make up the deeper tissues of the body. The epidermis is made of four to five layers of epithelial tissue that constantly grows from the inside out and replaces most of its cells every few weeks.

- The deepest layer of the epidermis is the stratum basale (stratum germinativum), a layer of stem cells that produce all of the keratinocytes, or skin cells, in the epidermis. If this layer is destroyed in a particular region, the epidermis cannot regrow and is replaced by scar tissue. Melanocytes found in this layer produce the pigment melanin to give the skin its coloration and protect the body from ultraviolet radiation. Special touch sensitive cells known as Merkel cells are also present in the stratum basale to detect the shape and texture of objects touched by the skin.- Superficial to the stratum basale is a thicker layer of epidermis known as the stratum spinosum, or spiny layer. Young keratinocytes are pushed into this layer from the reproduction of stem cells in the stratum basale. In the stratum spinosum, these keratinocytes begin to fill with keratin and form spiny connections between cells known as desmosomes. Desmosomes hold the cells together tightly while keratin fills the cells and gives the skin its strength and water resistance.- The next layer, the stratum granulosum, is a thin layer featuring granular keratinocytes that have been pushed out of the stratum spinosum. The granular appearance of keratinocytes is due to keratin and other chemicals accumulating inside the cells. Keratinocytes in the stratum granulosum also secrete lamellar granules, a waterproof substance that prevents water from leaking out of the body’s tissues or leaking into the body from the external environment.- Beyond the stratum granulosum the keratinocytes are cut off from their blood supply and begin to die, resulting in layers of dead keratinocytes on the body’s exterior. In the thick skin on the soles of the feet and palms of the hands the stratum lucidum is a thin, transparent layer of dead keratinocytes lying superficial to the stratum granulosum. The outermost layer is the stratum corneum, a thick layer of dead, flattened, keratin-filled keratinocytes that protect the underlying tissues. Dead keratinocytes slough off from the exterior surface of the stratum corneum only to be replaced by new cells emerging from the deeper layers.

Deep to the epidermis are the connective tissues of the dermis. Two major regions make up the dermis: the papillary layer (stratum papillarosum) and the reticular layer (stratum reticulosum).

- The papillary layer is the most superficial layer of the dermis; it contacts the epidermis. Between the epidermis and dermis it forms an undulating border that is rich in blood vessels and nervous tissue. These nervous and vascular tissues support the epidermis while the undulations, known as dermal papillae, increase the surface area of this border region.- The reticular layer makes up the majority of the skin’s thickness and is primarily made of dense irregular fibrous connective tissue. A thick web of collagen (the strongest protein found in nature) and elastin fibers makes up the dense irregular connective tissue and provides great strength and elasticity to the dermis. Many arterioles and venules also pass through the reticular layer to provide blood flow to the superficial tissues of the papillary layer and epidermis.

Deep to the dermis is the subcutaneous layer, also known as the hypodermis. The subcutaneous layer contains mostly loose connective tissues such as adipose and areolar connective tissue. These tissues store energy as triglycerides; provide insulation to the underlying tissues; and loosely connect the skin to the body. This loose connection increases the flexibility of the body, especially at the joints where a large range of motion is extremely important.

Being the largest and most superficial organ of the body, the skin provides many important functions to the body. The skin acts as the primary barrier to keep materials from passing into and out of the body. This barrier function prevents pathogens and toxins from entering the body and keeps vital fluids locked in the body’s tissues. The skin also protects the underlying tissues from UV radiation, extreme temperatures, and friction. Its high capacity for replacing itself results in the skin being able to sustain significant environmental damage and then quickly heal to resume its function. A dense network of nerves provides the skin with the ability to sense touch, pain, temperature and pressure from the external environment. Even the blood vessels in the dermis function in the regulation of body temperature by controlling how much hot blood passes near the body’s surface or is kept in deeper tissues.

The skin also contains many accessory structures that assist enhance its functions. Hairs growing out of the skin help to shade and insulate parts of the body to protect them from the environment. Nails reinforce the fingertips and can be used for scratching. A variety of exocrine glands also add function to the skin through their secretions. Sudoriferous (sweat) glands help maintain the body’s temperature homeostasis through evaporative cooling. Sebaceous glands help to lubricate and hydrate the skin while making it more waterproof. Finally, ceruminous glands produce earwax to protect the ear canals from pathogens and environmental pollutants.